Data protection plays an increasingly important role in today's computer systems. Data protection often involves creating regular backups of data. Data may be backed up in various ways, such as a full backup or a differential backup. A full backup typically backs up an entire disk, while differential backups typically backup blocks that have been changed since the previous full backup. It would be desirable to be able to record the blocks that have changed since the last full back, even if the system has rebooted.